The present invention utilizes a solid electrolyte sensor for detection of certain gases. The Nernst equation describes the behavior of sensing devices using solid electrolytes. When two media with different partial pressures, P.sub.1 and P.sub.2, of a particular substance present in both media are separated by a solid electrolyte (ionic conductor) and conducting electrodes are attached to both sides of the ionic conductor, an EMF is generated which is related to the partial pressures as follows: ##EQU1## where R is the gas constant, T is absolute temperature, F is the Faraday constant, E.sub.o is the standard oxidation-reduction potential difference, EMF is electromotive force, and n is the number of electrons per molecule of product from the overall cell reaction.
If the system described by the above equation behaves nonideally, the partial pressures must be replaced by fugacities. Another factor which may need to be considered in regard to a particular system is the rate of dissociation to form the ions which pass through the solid electrolyte. This may be a limiting factor to the transfer of ions through the electrolyte. The rate of dissociation can be calculated by means of the equilibrium constant for the dissociation reaction.
The magnitude of EMF produced is generally in accordance with the parameters discussed herein: the Nernst equation and, where applicable, the dissociation equilibrium constant. However, required practice in measuring concentration is to periodically calibrate the measuring apparatus by use of samples whose composition is known. Thus, exact adherence to theoretical relationships is not required of commercially used methods and apparatus. The primary commercial requirement is repeatability.
In a majority of cases, the admixture of an organic compound, especially in a polymeric state, with an inorganic compound, results in a phase separation, due to the fact that the two systems are immiscible in nature. However, we have discovered that a thin film polymer-blend membrane may be fabricated by admixing the organic and inorganic components discussed herein; the resulting substance is not merely a physical mixture but exhibits a degree of interaction, that is, some amount of chemical interaction exists. Substances which are permeable by gases in a selective manner are known and utilized in a variety of applications. A membrane formed in accordance with the present disclosure is substantially impermeable to ions and gases, including hydrogen gas, but does allow hydrogen ions to pass through it. For background information relating to the principles of the present invention, reference may be made to the book Solid Electrolytes and Their Applications, edited by Subbarao, Plenum Press, 1980.
Low mechanical strength has been a common problem when attempting to apply permselective membranes. The present invention provides a membrane whose mechanical strength is increased by compositing it with other materials, but whose desirable properties are not lost as a result of doing so.
Also used in the present invention is a solid substance which is a substitute for a reference gas, which reference gas is one of the two media mentioned above in the discussion of the Nernst equation. It is highly desirable to use a solid reference substance which requires only periodic replacement instead of maintaining a continuous reference gas flow, or in appropriate situations, maintaining a sealed chamber of reference gas. The reference substance is in intimate contact with the catalytic agent on the reference side of the membrane. One substance may serve as both reference substance and catalytic agent.